Radio frequency wattmeter



Sept. 2, 1952 H. C. EARLY RADIO FREQUENCY WATTMETER Filed April 30, 1946FIG.I

l/ I IO I I I I /l/l//l//ll/ /NVENTOR FG. 3 HAROLD c. EARLY ATTORNEYPatented Sept. 2, 1952 UNITED {SI -T ES ?Nr-;Elv r OFFICE v j RADIOFREQUENCZWATTMETER'; 7 Harold o. Easy; jn' Arber; Mick; assignor'to the'United States of Americaas represented by' u u the SecretaryofWarApplication April 30, 1946, Serial No. '665,'991

` 4 Claims. (c. iva-44 g --This invention relates 'generally toelectrical apparatus and more particularly to a directreading wattmeteradapted, to showpower consumed in a load fed througha waveguide.

,In other methods of-determining the power consumed in systems employingwaveguide transm'ssion the readings which are obtained due to the RFfield within the guide cannotbe -used without corrections to compensatefor: thestand ing wave. ratio in the guide; changes in operatingfrequency; and the *positions of .the pickup element inside the guidewith respect to points of maximum and minimum voltage.

One method havingcertain advantages but nevertheless having limitedusefulness is indi-. cated n:Fg. 1 of the drawing. By the use ofthisarrangment it is possible to, distinguish between radio frequencyenergy traveling through the-guide in, opposite directions Obviously anyRE energy moving from left toright in the guide as shown in Fig; 1;-reaching hG WQ DIQbGS which I are separated by ;a quarter wavelengthcorresponding-;to the Operating frequency and encountering' a system-inwhich the second of these probes feeds aline which is longerbyonequarter wavelength than theline fed by the first probe, will haveno'effect in a thermo couplefed by both of these lines at a junction attheir *terminations. This is due to cancellation of two voltages 'pickedup by these probes, and this cancellation results from their ISO-degreephase dii ference caused by a total of one-half wavelength of delay ofone of them. It is equallyobviousvthat energy moving in'the op positedirection' in the guide, and encountering the two probesin conversesuccession, will add at the 'thermo coupling. Therefore, the apparatusshownin Fig. 1 has the quality of being selective as tothe direction ofthe traveling waver r i However, the -function of: this device will beadversely efiected by ichanges in op'erating'frequency; Moreover; thoughit can give an indication proportional to the energy moving in onedirection, itcannot, by itself, give simultaneous indications of boththe forward and 'the reflected energy nor of the difference energybetween them.

It is an object of this "invention to' provide a direct'readngwattmeter-:which will be selective as to the direction of movement of anjRFwave traveling in a waveguide; It is'. a further object, ofthisinvention that the apparatus must' not be frequency sensitive; :Itis afurtherobjectof this invention that' a single devic'e .will be capableof presenting simultane'ously ;indicator readings proportional to RFpowerv -levels of both the forward and the reflected energy in the waveguide and of the difference in power levels between themthat is thepower consumed in the load. It is also an object of this invention thatit should operate satisfactorily without the necessity of the pickupelement being inserted into' the guide at any particular point wthrespect to the points o voltage maxima and minima.

Generally', this invention comprises a pickup element which is selectiveas to the direction of propagation of RF energy moving in a waveguideand. which is capable of delivering two output indicationssimultaneously, one proportional to forward power in the guide and'theother proportional to refiected power. I

Other objects, features and advantages of this invention will' suggestthemselves to those skilled in the art and will become apparent 'fromthe following description of the invention taken in connection with the'accompanying drawing in which: x r i Fig. 1 is a diagrammatc sketch ofthe layout of a radio frequen'cy energy indicating apparatus which isselective as to the direction of a traveling wave but whichhas'shortcomings which hav been overcome in this invention; i V y Fig.,2 is a transverse sectional View of a standard type of rectangularwaveguide, said sectional View being taken along line 2-2 of Fig. 3.-'This section outs across the-waveguide itself .and the tapered rampridge installed therein. This figure' shows, notin section, an` end Viewof afragment of the supporting means for the pickup element and showsthe relative locations of theelements of this apparatus for the TEo,1mode of operation. Fig. -3 is a longitudinal sectional View of afragment of the guide shown in Fig. 2, 'said View being taken along theline 3--3 of Fig. 2. This section passes through-the 'plane where thetwo halves of the supporting means for the pickup element 'abut against.*one together. v I

Referring more particularly'to Fig. 3, guide I, having opposite sides 2and 3, has a conductive tapered ramp ridge .4 attached longitudinallywithin it alongthe center of the inner surface of side 3. This elementis designed according -to general practice governing the design ofwaveguide capacitance loading elements. Obviously its taper should begradual enough and its length sufcient, 'with respect toone wavelengthcorresponding to the lowestintended Operating fresquency, so that thediscontinuity occasioned'by its lumped `'capacitance is introducedgradually,

andso that objectionable reections do not occur.

another and are fastened t ment 5. The bottom surface of element is they surface which abuts against the outside of side 2 of waveguide l atthe point of installation. This extension is formed so as to fit tightlywithin an opening in side 2 and a 'smallarea n atthe end of thisextending portion replaces a section of the inner surface of side 2which is cut out to permit the insertion of pickup element 6 and theinstallation of supporting element 5.

pportv ing element 5 may be attached to side 2 by any convenient meanssuch a'screws.

' Halves 'l and 8 each have grooves of semicircular cross section cutinto them'so that when these halves are assembled together twosubstantially cylindrical or tubular channels extend from two of'theopposite sides of supporting element 5; through element 5; to adi'acentopenings at the center of area 10.- Th'e diameters of these cylindricalorifices are equal 'and should be such that they will permit, withinthemselves, a snug fit of 'elements comprising sections of the centerconductors with their surrounding insulating sheaths, of two lowimpedance, high loss, coaxial cables'` These cables may be of standardtype and are described more fully hereinafter.

It will be seen by reference to Fig. 3 that the cables shown thereinconsist ofninternal conductors'l l, dielectric sheaths !2, whichsurround internal conductors Il; external conductors !3, which,-` 'asin'common practice, may be braided fiexible conductors or shields';"andouter coverings M of protective material which may be 'dielectric.

Protective' coverings M in this arrangement have been'cut back so asto'exp'os'e considerable lengths of 'dielec'tric sheaths !2 and ofbraided shields l3. Braided'shieldsl, in turn, are cut back on sheath12. Receptacles !5 are mechanically and electric'ally joined to the thusexposed outer conductors !3. Receptacles [5 are similarly attached L totwo opposite sides of supporting means 5 directly over the places Wherethe two cylindrical channels enter element 5 so that the exposedportions of Shea-ths 12,'a1ong with inner conductors H which theycontain, extend into and through these channels. v

As a result'of this "arrangement, the coaxial lines are electrically'continuous' despite their junctions to' element 5 and they'will enterwaveguide 'I side by side in the center of' area ID of theextended'portion'of supporting element 5.

^ At the point where' inner conductors l I emerg'e near the center ofarea !0 they are mechanically and lectrically joined to opposite sides'of small piokup element 8-. This element, which is intended to serveboth as a capacitive' pickup and an inductive on'e, has a shape somewhatlike a very short tube 'which may be substantially oval, rectangular, orcircular' in' cross section, which is open along its side whiclrisnearest to area ID by 'an opening which has'theform of a slot runningalong said side of said tube and parallel to itsaxis. -The axis ofthishypothetical tube is atright 'angles' to' the axis of the waveguide.

' Regarded in this manner, it becomes apparent that there is' more tothis loop'than a loop which would be formed by bending a thin wire intothe form of a small, almost-closed circle. Instead, this loop hasappreciable width relative to its other dimensions and, therefore, itsside farthest from area n presents an appreciable area in a planeapproxmately parallel to area ID. This area, therefore, is roughlynormal to the electric field 'within the guide, for-ethe TE,mode, and,as a result, element 6 is-` adapted to` serve, at least in part, as acapacitive pickup.

For the purposes of convenience hereinafter, the coaxial line, a portionof which is shown on the left of supporting element 5 in Fig. 3, will bereferred toas coaxial line I B, and the coaxial linega portion of whichis shown correspondingly attached to'the right of supporting element 5,will be, referred to as coaxial lin H. coaxial I lines !5 and !1 shouldbe low impedance, high loss lines; equal in length; identica in theircharacteristics; and of such length that the small amounts of energypicked up on element 6- and conveyed from it to the entrances 'ofcoaxial lines IE and IT, will be a-dequate to reach the terminations ofthese lines despite losses, but will not be adequate, even if onehundred per cent reflection occurs at these terminations, to get back,in other than negligible amounts, totheir common point of origin atpickup element 6. Across the far terminations of coaxial lines !6 and llthere should be attached devices, such as thermocouples, which arecapable of processing the RF energy reaching them so that they canactuate indicating means and produce indications proportional to theamounts of RF energy reaching said devices. These two devicesshould haveOperating characteristics as nearly alike as possible.

In operation direct power traveling along guide i, from the sourcetoward the load, will'encounter element 6. The presence of this element'will cause certain voltages to appear atthe entrances, or input ends, ofcoaxial lines !6 'and IT. A voltage capacitively picked up by element 8'will be conveye'd to the two inner conductors ll as two voltages whichwill be in phase. This phase'relationship is due to the fact that pickupelement 6 is very small with respect to one wavelength corresponding toeven the highest intended operating frequency and that, therefore, thevoltage along its small length will be substantially the same at anypoint thereon. However. the voltage inductively picked up by element 6will be conveyed to the entrances of coaxial lines 16 and H as twovoltages' degrees out of phase' to eachother. One of these two voltagesof inductive origin will be in phase with both of the voltages ofcapacitive origin while the other voltage of inductive origin will be'180 degrees out of phase'with both of them. Therefore, the voltage ofcapacitive origin in one of 'the coaxial lines may be increased by oneof the voltages of inductive origin; whereas, the voltage of capacitive'origin on the other coaxial linewill be rereduced 'by the other voltageof 'inductive origin. If quantitively the magnitudes of the voltages ofcapacitive origin and those of in'ductive origin are caused to be equal,it follows that there will be a doublng of the RF voltage in one lineand complete cancellation of the RF voltages in the other.` Thisbehavior on the part of pickup element e is` directly eifected by thedirection in which the traveling wave moves'in the guide. Therefore,if'it should happen' that there is RF vltage cancellation on coaxial'line 16,' and RF voltage doubling'on coaxial line H, when element6is-exposed only to forward power propagation en em inthe guide (thatis, when thereis complete freedom `from reflection, .a :condition whichwill exist .if .the guide is, in effect, infinitely long' or if it isterminated'in 'a perfectly matched load), then these voltage conditionsonthese `two lines would be exchanged one 'for the other,ifz thedirection J of power 'propagation were `reversed by exchanging thepositions. of the generator and theload. I r' If,however, .reflectionof: RF' energy from the load side of the'system is permitted to occur,then neither line will have a zero .RE output. In'- stead, additive .RFvoltages will simultaneously occur on both coaxial linesi 1 6 and .H.'The total RF energy appearing-on one .of .these eoaxial lines will 'beproportional only to the radio' frequency energy in the forward movingwave; while that appearing on the other :will be proportional'only tothe radio frequency energy in the' reflected wave; a t

It is obvious that the different outputs *existing between the twothermo couple outputs at the ends of these two lin'es 'will be:proportonal'to the' power usefully consumed in the load plus any smallamount :of power 'lost-'in the-transmission system between 'the pickuploop. and the load. However, losses in waveguide systems are frequentlynegligible. Therefore this device, in such instances, will'yield'difference output rea'd ings (on the `indicating device used) which'can be considered as substantially *proporti'onal to only the powerwhich 'is usefullyconsumed.

'It has been empirically established that pickup element G'can be'proportioned so' that the voltags picked up by it capacitively andinductively are equal in magnitude. This is done by proper selection ofthe relative areas of the cross section of the loop and of the closedside of the loop which is substantially parallelto area n. Anotherfactor which bearsupon achieving this result is the conditionof'rel'ative strengths of the magnetic and electric fields existingwithin the guide at the pointof pick-up; In this connection it has beenfound desirable to' locate the'pick up element near an edge of side"Zr'a'ther than near its center( This -a-llowsthe selection of rationalv sizes 'for the dimensions of the` pick up loop. Moreover, there`exists inthis part of theguide the desired phase relationship betweenthe electric and magnetic Vectors. 4 also eects the ratio of relativemagnitudes of the electric and magnetic Components of the RF field inthe guide. For this reason it plays a part in achieving the desiredresults described above. Ridge 4, however, also serves an added purpose.It causes the waveguide to pass a frequency band which is wider on thelong wavelength side of the pass band. That is, it reduces the lowfrequency cutofi frequency of the guide. It has been found that thiselement, at the same time, tends to maintain the magnitudes of themagnetic and electric Components at a relatively constant ratio over thethus increased Operating band width of the guide. For this reason, theassociated use of ridge 4 makes it possible for this apparatus to remainin proper adjustment and to give useful indications when the system isOperating over a band of frequencies rather than at a singlepre-selected frequency.

There is another reason for locating the pick up element near to an edgeof side 2 and, as well, at a certain particular distance from it. It isthe need to compensate for a small phase shift produced by the uncoupledself-inductance of the loop. Since there is some capacitance betweenTapered ramp ridge the loop and side 2 of theguidethis `capacitance canbe adju'sted to compensate for the phase shift referred to above; Thus,the exact desired'phase relation described above will exist. 4

When :the` frequency' of' the energy passing throughwaveguide'-.l-;is'inceased, a change occu's *which can' be easily'described asan increase in the sensitivity ofthe pick' uprloop..:In'other Words, the voltages picked up fromzboth'thetelectrio ;and themagnetic fields'in the zguideincrease simultaneously. However if the'adjustm'ents: al ready described :have beenmade land if` the gen-'i eraldesign :already described has been followed; the ,ratio betweenthese-voltages .which are 'picked up capacitively andinductively will;remain 'the i same `despite their increases in magnitude. Because ofthis, and. if the guide impedance :remains substantially constant; thedirectional properties of thisdevice will nctbe disturbed by; increasesV in frequency within `'certain limits. 1, These 'increasesin:voltage,1however, :will effect the' cali` bration of;.theoutputindicating devices. But. thiseect can begelirninated without ;muchtrouble.: It hasbeen found thatthelosseswhich occurmin-coaxial tlinesliand lli'increase for in-j creases in frequency and can be znade to-doseat approximately the n ecessaryrate to roughly. compensate fothesfmultaneous .changes in the sensitivity of the loop; The net' resultcan ;be caused to be such `that the -calibr:ation;;o f; :the powerindicatonwin beqaccurate -through' :the

wholefrequency range of cperatienwithout.need

for ;readjustment or fa'trsv p .Calibration of thisdevice can be Lcneved by conventional means,` -An example ef one method w l l ee t iniet k wn a ane &radioree quency energy into the input; end otthe guide; toadjust the; system for unity standing wave ratio ;2 and, under theseconditions; te calibrate the indicator (which of course, is connectedto. the appropriate thermo couple which, in turn, is attached to theappropriate coaxial. 1ine for ,the use of: corrective lln this wattmeteran air thermo' couple should prefrablybe used ,ln an air thermo couplethe'heatlo'ss is .mainlyby ccnduction tc-. the sup ports andbyConventionso the :direct our.:

rent output is -very'c1osetojthe RF power onto the square of the RFcurrentthrough the thermo couple. It is essential that this condition beobtained if the two thermo couple voltages are to be combined to make adirect reading instrument. In this connection it would bewell to bear inmind that most thermo couple meters use a vacuum type thermo couplewhere the heat loss is mainly due to radiation and the relation betweenRF power and the direct current output is far from linear. Therefore iftwo air thermo couples and D. C. meters are used, the meter Scales canbe calibrated to read directly in watts or the scale of a single D. C.meter can be so calibrated if the apparatus is arranged to use one meterwhich is actuated by the difierence voltage.

It is obvious in view of the principles which govern the operation ofthis device that its function in no way depends upon the location of thepick up 'element with respect to the points along the guide wherevoltage maxima and minima occur.

It will be obvious to those skilled in the art' 2, &09,450

ployed at the terminations of the high loss lines.

This freedom of selection is available without departing from the basicprinciples of this invention. V

Accordingly I claim all 'deviations which fall fairly within the spiritand scope of the invention as identified in the hereinafter appendedclaims.

What is claimed is:

1. A wattmeter for` indicating the radio frequency power consumed-in aload ed by a waveguide, including a 'rectangular waveguide fortransmission of electrcmagnetic energy in the TEo,1 mode, capacitive andinductive conductive pick-up means in the form of a tubular partial loophaving a diameter which is a very small p fraction of one wavelength of,the highest intended Operating frequency and having a substantialextent in a direction normal to said diameter, means supporting saidloop inside said waveguide in a, plane parallel to the lines of electricforce between opposite sides of said waveguide and to the axis of saidwaveguide, said extent of said loop being substantially perpendicular tosaid lines of electric force and 'said loop being located within saidwaveguide between an edge and the center of one of the two sides betweenwhich the electric field extends whereby voltages capacitively andinductively picked up within said waveguide by said loop are of equalmagnitude and are either in phase or 180 out of phase at the oppositesides of said loop, two radio fr'equency transmission means electricallyconnected to the two opposite sides of said loop and disposed totransmit radio frequency energy transferred to them from said loop topoints outside of said guide, said transmssion means having likecharacteristics and substantial attenuation.

2. A wattmeter as in claim 1 in which a capacitance loading element isadded to the apparatus, said loading element being mechanically andelectrically attached to the inside of said waveguide to extend thelower cutofi frequency.

3. A wattmeter for measuring radio frequency energy comprising awaveguide of rectangular cross-section for the propagation ofelectromagnetic energy, a tubular pick-up element having a substantiallyfiat portion and partially looped portions, means supporting saidpick-up element in an oli-center position in said waveguide with saidflat portion normal to a component of the electric'field of saidelectromagneticenergy and said partially looped portions normal'to' acomponent of the magnetic field' of said electromagn'etic energy, and apair of means coupled to' opposite sides of said partially loopedportions for obtaining a pair of signals therefrom wher'eby said signalsgive an indication of the electromagnetic energy propagated in saidtransm'ission means in one direction and' inithe reverse' direction,respectively. i' e y fi. A wattmeter for measuring radio frequency powercensumed in a load ed' by a, waveguidevincluding a rectangular waveguidefor transmission of electromagnetic energy in the` TEo, 'moda capacitiveand inductive pick-up means having the form of a cylinder with anaperture parallel to the axis thereof and having a diameter which is aVery small fraction of one wave-length of the the highest intendedOperating requency, means supporting'said pick-up means inside said'waveguide adjacent one of the two wider sides of said rectangularwaveguide and between the center and the edge of said one side and `withthe axis of said cylindrical pick-up means parallel to said one side andperpendicular to 'the two shorter sides of said rectangular waveguide,two'radio frequency transmission means coupled, 'to said cylinder onopposite sides of saidaperture and disposed to transmit radio frequencyenergy from said pick-up means to points outside of said waveguide, said'transmission means having like characteristics andsubstantialattenuation. r

HAROLD C. EARLY;

REFEREN CES CITED The following references are of record in the file ofthis patent:

Crosby July 8, 1948

